1. Field of the Invention
The present invention relates to a solid electrolyte type fuel cell.
2. Description of the Prior Art
Recently, fuel cells have been noted as power generating devices. The fuel cell is a device capable of directly converting chemical energy included in a fuel to electrical energy. As the fuel cell is free from limitation of Carnot's cycle, the cell is a very promising technique because of its inherently high energy conversion efficiency, wide latitude of fuels to be used (naphtha, natural gas, methanol, coal reformed gas, heavy oil and the like), less public nuisance, and high electrical power generating efficiency without being affected by the scale of installation.
Particularly, as the solid electrolyte type fuel cell (referred to as "SOFC" hereinafter) operates at high temperatures such as 1,000.degree. C., activity of electrodes is very high. Moreover, the SOFC has low polarization and relatively high output voltage without requiring any catalyst of an expensive noble metal such as platinum so that energy conversion efficiency is much higher than those of the other fuel cells. Furthermore, the SOFC is stable and has long service life because all the constituent materials of the SOFC are solid.
One example of such solid electrolyte type fuel cells is illustrated in the partial front view of FIG. 1 and partial perspective view of FIG. 2. This is a SOFC of a monolithic design, referred to as "Co-flow Model" of Argonne type which was initially proposed by the Argonne National Laboratory.
In the SOFC of this type, as shown in FIG. 1, a flat plate-shaped air electrode film 44, an interconnector 43 and a flat plate-shaped fuel electrode film 42 are laminated from above to below to form each of flat plate-shaped laminates 41. The plate-shaped laminates 41 are then arranged in parallel with one another with a predetermined interval. A number of fuel electrode films 52 having a substantially V-shaped section as shown in the drawing are arranged in opposition to the flat plate-shaped fuel electrodes 42 to form a number of fuel gas passages 20 perpendicular to the surface of the drawing of FIG. 1. Moreover, a number of air electrode films 54 having an inverted V-shaped section are provided in opposition to the flat plate-shaped air electrode films 44 to form a number of oxidizing gas passages 30 in the same direction of the fuel gas passages 20.
These fuel gas passages 20 and the oxidizing gas passages 30 are combined with one another in the form of a mosaic so that wave-shaped solid electrolyte films 53 are formed between the fuel electrode films 52 and the air electrode films 54. With this arrangement, between the adjacent fuel gas passages 20 and oxidizing gas passages 30 there are interposed the fuel electrode films 52, the wave-shaped solid electrolyte films 53 and the air electrode films 54 in this order. The power generation is performed in these interposed films. Although the films participating in the power generation are shown only in one row for the sake of simplicity in FIG. 1, a number of laminates shown in the drawing are laminated to form a number of gas passages in the form of a honeycomb.
In generating electric power, a fuel gas is supplied into the inlets of the fuel gas passages 20 as shown by arrows C in FIG. 2. The fuel gas flows through the fuel gas passages 20 and are exhausted from the outlets of the fuel gas passages 20 as shown by arrows D. At the same time, an oxidizing gas is caused to flow through the oxidizing gas passages 30 as shown by arrows E. The oxidizing gas and the fuel gas flow in opposite directions, respectively. In FIG. 2, the respective electrode films and the laminated components such as the inter connectors shown in FIG. 1 are not shown for the sake of clarity.
With such a SOFC described above, there has been a problem of steep temperature gradient due to concentration gradient of fuel gas streams. In more detail, when the fuel gas flows into the fuel gas passages 20, a relatively large amount of fuel is consumed for the electrochemical reaction to raise the temperature in the proximity of the inlets of the fuel gas passages 20 because the fuel gas contains yet abundant amounts of the fuel near the inlets. As a result, the electrochemical reaction of the fuel with oxygen ions at the fuel electrode films 52 is more and more activated by the temperature rise.
On the other hand, the fuel gas flowing through the fuel gas passages 20 reduces its fuel concentration as the fuel gas approaches the outlets of the fuel gas passages 20 so that the amount of the fuel consumed in the electrochemical reaction progressively reduces. Therefore, the temperature of the fuel electrode films 52 does not rise sufficiently and hence the electrochemical reaction becomes more inactive. Moreover, the fuel gas of the reduced concentration includes fairly large amounts of CO.sub.2, moisture and the like which would attach to the surfaces of the fuel electrode films 52 to impede the electrochemical reaction thereat, so that the reaction becomes more and more inactive.
Consequently, steep temperature gradient is caused between the upstream side and the downstream side of the fuel gas. When the SOFC is operated in such a steep temperature gradient for a long period of time, cracks tend to occur in walls of the passages and electric power generating efficiency itself is detrimentally affected.
While the problems of the monolithic type SOFC have been explained, the same holds true in the conventional flat plate type SOFC.